The use of lasers for the transmission of information in communication systems is known. Such use has typically been limited to amplitude or phase modulated systems that in use often attain a speed of 1.5 gBPS.
Laser systems in current use for communications (e.g., a solid state pumped DFB 1550 nm lasers with 100 mW output available for fiber optic communication system from Laser Power Corp., San Diego, Calif.) are typically amplitude modulated because of certain inherent limitations in a laser's ability to change frequency. Lasers, in fact, are often limited to a single frequency or a narrow range of frequencies (typically .+-.60 kHz). The ability of a laser to produce a stable optical carrier frequency is inherent in the resonant cavity used in the generation of laser signals.
A resonant cavity of a laser is designed to amplify optical signals of a desired frequency and attenuate signals of an undesired frequency. The cavity amplifies desired frequencies through use of a laser cavity dimensioned in one-quarter wavelength increments. The closer the cavity dimensions are to a desired tolerance, the narrower the range of frequencies within which the laser will operate (the linewidth). The narrower the linewidth, the less inherent phase noise will be transmitted in a laser signal. Further, the narrower the linewidth, the more power is focused into a desired center frequency.
While amplitude or intensity modulation is the predominant mode of data transmission, some progress has been made by using a combination of AM modulation and multiplexing. For example, systems have been proposed using subcarrier multiplexing (SCM) of the laser beam as an alternative (e.g., Goviad P. Agrawal, Fiber Optic Communications Systems, 2nd Ed., Wiley Series in Microwave and Optical Engineering, Kai Chang, Series Editor, Chapter 7, Section 7.5, pgs 335-345, John Wiley & Sons, Inc. New York 1997 ISBN 0-471-7540-4; Leonid Kazovsky, Sergio Benedetto and Alan Willner, Optical Fiber Communications Systems, Arttech House, Inc., Norwood, Mass., 1996, ISBN 0-89-006-756-2, Chapter 4, Sections 4, 4.1, 4.2 and 4.3; etc.).
Because a laser is essentially a resonant cavity, it tends to resist changes in frequency. Direct amplitude modulation by current injection has been the preferred method used in prior systems, thus modulating the laser beam at its source. This has proven only partially successful because of a tendency of the resonant cavity to exclude frequencies outside a low bandwidth regime. Direct AM modulation is typically limited to an information block of less than 1.5 GHz in spectral bandwidth. Because of the importance of optical communications systems a need exists for improved methods of optical modulation.